Second, we can rule out the types of stars that are unlikely to lead to technological civilizations. A star only 2 times

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Second We Can Rule Out The Types Of Stars That Are Unlikely To Lead To Technological Civilizations A Star Only 2 Times 1 (167.21 KiB) Viewed 25 times

Second, we can rule out the types of stars that are unlikely to lead to technological civilizations. A star only 2 times more massive than the Sun lives only 1 billion years - much too short given that it took 4.5 billion years for us to appear on Earth. At the same time, stars much smaller than the Sun (red dwarf stars) produce violent outbursts and may wipe out life before it can evolve. For this calculation we will restrict ourselves to stars pretty close to the Sun in mass. That means other G-type or G dwarf stars. You can find the number of G dwarfs in the Milky Way at this website. Look for "Fraction of Stars" in the "G dwarf” row. Multiply the number of stars in the Milky Way by these two fractions: the fraction of stars in the right part of the Milky Way and the fraction of stars that are G dwarfs. This gives the number of suitable stars in the Milky Way S* = The ne parameter This is the fraction of stars with habitable planetary systems. We are looking for “life as we know it”, which means chemical life, and this probably means that we need liquid water. We can imagine life forming in liquid ammonia, or other such exotic fluids, but let us be conservative for now. ne become the number of planets in the star's “Goldilocks zone” or “habitable zone” (where water can be a liquid), for a star that has a planetary system. Until relatively recently, this was almost completely unknown. We can now make a good estimate of this based on the results of the Kepler mission. This paper makes an estimate of the average number of rocky planets per star that are not too much larger or smaller than the Earth, and which orbit in the habitable zone of their star. It makes this calculation for stars not too far from the Sun's mass, so it is appropriate to use this for our G dwarfs.
Note that this paper gives a range of values that depend on the assumptions that went into their calculation. We are going to record both values so we can calculate and upper and lower limit for our final Drake equation number. maximum np = minimum n = The fi parameter It is very difficult to estimate the fraction of “Goldilocks” planets that go on to develop life. We really only have our solar system to go on. Of the two such planets in our system, one developed lasting life (if Mars has life, it is very scarce and not really worth counting). But there is a huge observation bias (obviously life exists in the only solar system that we can observe), and so it is not clear that "50%" (or 0.5) is the right number. On the other hand, we know that life developed very quickly after the Earth became habitable; only 300 million years: a blink of the eye in its 4.5-billion-year history. This suggests that life might be pretty common. Discuss this with your group and with the class and come up with a reasonable range for the likelihood of the formation of life. You should justify that range! maximum f = minimum fi= We now have enough parameters to make a calculation. We still cannot figure out the number of technological civilizations that we could communicate with - we will get to that next week. However, we DO have enough information to estimate the number of life-bearing planets in the galaxy. That is just the first half of the Drake equation: Nlife= S+. Np.fi Calculate the Drake equation two times: first using your values for the maximum fo and figuess , then using their minimum values. This gives you the range that you might expect the true number of life-bearing planets to fall in. maximum Nlife = minimum Nlife = Does this range sound reasonable? If not is it more or less than you expected? Based on this range, do you expect there to be many or very few technological civilizations in the Galaxy? For this last question just make a guess - we will do the proper, full Drake equation calculation next time.
The overall number density of stars in the solar neighborhood is approximately 0.0984+0.0068 star/pc^3 (i.e. ~0.10 stars per cubic parsec), including white dwarfs, but not including brown dwarfs, neutron stars, or black holes. Approximately -0.092 star/pc^3 of this number density is just M dwarfs, with the remaining -0.026 due to all other types of stars. Hence, M dwarfs represent approximately –78% of all stars (depending on how it is calculated, this number seems to consistently come up in the 70-80% range). Star Type O stars B stars A stars F stars (all lum classes) # Density (pc^-3) Fraction of stars Reference Comments #1: cross-referenced GOSS (Sota+2014) catalog w/van Leeuwen(2007) & Gaia DR1 TGAS. N=5 O-type stars w/ ply --5e-7 4.4e-8(+2.0e-8)[#1] EEM > 3.33 mas (d<300pc): zeta Oph, delta Ori, zeta Ori, 15 Mon, HD 37737. Parallaxes very poor beyond that for most (1 in ~2,000,000) stars. 3.9e-4 3.2e-5(=4.7e-6) (#1] EEM #1: calc from HIP2 (B-type, Mv < 2, plx/e(plx)>8, d<70pc, N=46) (1 in 2600) 6.0e-3 4.9e-4(+6.6e-5) [#1] EEM (1 in 170) #1: calc from HIP2 (A-type, My<4, plx/e(plx)>8, d<30pc, N=55) 0.0025(+0.0003) 0.031 EEM #1: see http://www.pas.rochester.edu/-emamajek/Fstars_dens.txt [#1] (1 in 33) 0.0024(+0.0003) 0.029 EEM [#1] #1: see http://www.pas.rochester.edu/-emamajek/Fstars_dens.txt (1 in 34) 0.0048(+0.0011) 0.00332(+0.00044) 0.059 #1: calc from RECONS (10pc, N=20, likely complete) [#1] [#2] (1 in 17) #2: EEM SIMBAD plx>62.5 (d<16pc) (N=57), vetted for binarity, best SpTs, junk parallaxes. 0.0033+0.0004) [#1] #1: calc from Mamajek+08 (16pc, N=57, likely complete) 0.0038(+0.0010) 10.041 #2: calc from Mamajek+08 (10pc, N=16, likely complete) EEM [#2] (1 in 25) #3: calc from Kirkpatrick+12 (8pc, N=8, complete) 0.0037(+0.0013) Note: del Pav (6.1pc) classified G8IV, but within 0.8 mag of MS (so "dwarf") F dwarfs G stars (all lum class) G dwarfs [#3] 0.0105(+0.0016) [#1] 0.0135=0.0025) [#2] 0.01537=0.0054) #1: calc from RECONS (10pc, N=44, likely complete) #2: calc from Kirkpatrick+12 (8pc, N=29, likely complete) #3: calc from Kirkpatrick+12. (5pc. N=8. likely complete) 0.129 (1 in 7.8) K dwarfs EEM
K dwarfs 0.129 (1 in 7.8) EEM 0.0105(=0.0016) [#1] 0.0135(+0.0025) [#2] 0.0153(+0.0054) [#3] 0.0083(+0.0007) [#4] 0.0676(+0.0040) [#1] 0.0859=0.0086) #1: calc from RECONS (10pc, N=44, likely complete) #2: calc from Kirkpatrick+12 (8pc, N=29, likely complete) #3: calc from Kirkpatrick+12 (5pc, N=8, likely complete) #4: all K stars: calc from SIMBAD (16pc, N=143, 7 are evolved; looks incomplete for dwarfs) [#2] 0.725 (1 in 1.38) EEM #1: calc from RECONS (10pc, N=283, 2018.3) #2: calc from Kirkpatrick+12 (6.527pc, N=100) #3: calc from Kirkpatrick+12 (5pc, N=48, likely complete) #4: calc from Winters+19 (5pc, N=48, likely complete) M dwarfs 0.0917(+0.0132) [#3] 0.0917(+0.0132) [#4] 0.0048(+0.0011) [#1] 0.0051(+0.0015) [#2] white dwarfs 0.0048(+0.0005) (e.g. DA, DB, DC, [#3] DQ, DZ, etc.) 0.0055(+0.0001) [#4] 0.00449(+0.00038) [#5] 0.059 (1 in 17) EEM #1: calc from RECONS 10 pc sample (N=20 as of 2012.0) #2: calc from Kirkpatrick+2012 8 pc sample (N=11) #3: Holberg+ 2016 #4: Munn+ 2017 #5: Hollands+ 2018 "Evolved stars" 8.8e-4 0.011 (1 in 93) EEM includes stars just leaving main sequence, subgiants, giants, calc using HIP2 for Mv> 1 mag away from Wright 2004 main sequence, Mv <4.5 (d< 35 pc; N=158) "Red Giants" 2.7e- 44.9e-5)[#1] 3.3e-3 (1 2001 EEM #1: G/K/M giants only, N=31 w/id<30pc (using van Leeuwen 2007 HIP parallaxes)
0.09170.0132) [#4] 0.0048(+0.0011) |[#1] 0.0051(+0.0015) |[#2] white dwarfs 0.0048(+0.0005) (e.g. DA, DB, DC, [#3] DQ, DZ, etc.) 0.0055(+0.0001) [#4] 0.00449(+0.00038) [#5] 0.059 (1 in 17) EEM #1: calc from RECONS 10 pc sample (N=20 as of 2012.0) #2: calc from Kirkpatrick+2012 8 pc sample (N=11) #3: Holberg+ 2016 #4: Munn+ 2017 #5: Hollands+ 2018 "Evolved stars" 8.8e-4 0.011 (1 in 93) EEM includes stars just leaving main sequence, subgiants, giants, calc using HIP2 for Mv > 1 mag away from Wright 2004 main sequence, Mv <4.5 (d< 35 pc; N=158) "Red Giants" 2.7e-4(+4.9e-5)[#1] 3.3e-3 (1 in 300) EEM #1: G/K/M giants only, N=31 w/i d<30pc (using van Leeuwen 2007 HIP parallaxes) all stars OBAFGKM *s+ WDs ignoring NSs & BHs 1 EEM 0.0902(+0.0046) [#1] 0.0936(+0.0134) [#2] 0.0984(0.0068) [#3] # Density (pc^-3) #1: calc from RECONS 10 pc sample (N=378; 2018.3) #2: calc from Kirkpatrick+2012 5 pc sample (N=49) #3: calc from Kirkpatrick+2012 8 pc sample (N=211) Star Type Fraction of stars Reference Comments